a) Field of the Invention
The invention is directed to a process for the production of a device which is suitable for holding semiconductor disks (wafers) during diverse, preferably thermal, machining processes and for transporting semiconductor disks between different machining stations and which is formed of a plurality of individually pre-manufactured device parts that are assembled and connected with one another. The invention is further directed to a holding device for semiconductor disks which is manufactured by the production process according to the invention.
The field of use of the invention is semiconductor technology, especially wafer machining and wafer handling. In this connection, a plurality of semiconductor disks or wafers which are oriented essentially with surfaces parallel with one another and from which, for example, integrated circuits are to be manufactured in subsequent production processes are held in a holding device (wafer boat) and subjected to individual machining steps in this holding device, wherein the semiconductor disks and the holding device are subjected to high thermal loads among others.
This is carried out in clean rooms within an atmosphere of high purity which is free of foreign substances for extensive prevention of uncontrolled and unwanted deposits of extraneous atoms or molecules in the wafer material.
b) Description of the Related Art
It is already known to produce a holding device of the type mentioned above for semiconductor disks made of quartz or sintered ceramic material by fusing or sintering individual device parts. This is described, for example, in DE 39 07 301 A1, page 1, lines 15 to 22.
Because of the expenses incurred in production, the required precision, the considerable outlay on repairs in case of breakage of materials, and so on, it is suggested in the above-cited reference to assemble the holding device by means of detachable connections of the individual device parts.
A holding device corresponding to the above-mentioned prior art is shown in FIG. 1. In this case, four holding elements 1 in the shape of round bars are provided at their side facing the semiconductor disks with notch-like or groove-shaped recesses 2 in which the semiconductor disks (not shown in the drawing) can be set in such a way that they are held at four areas of their outer circumference in vertical orientation parallel to one another at a distance required for the relevant machining step.
Further, two front or end parts 3 are provided with surfaces, for example, for setting up the holding device on a base. There are openings 4 at the front or end parts 3 for fastening the holding elements 1, wherein the positions of these openings 4 determine the spatial arrangement of the holding elements 1. The holding elements 1 have, at their ends on both sides, threads 5 which, together with nuts 6, serve for connecting with the front or end parts 3.
It is disadvantageous that connections of the type mentioned above do not satisfy requirements for stable, positionally accurate holding of semiconductor disks especially in the high temperature range, above all with respect to the considerable weight loading due to the often large number of semiconductor disks to be held. Further, while holding devices made of quartz do meet purity conditions, they are resistant to deformation only up to a temperature of 1057.degree. C. and, in the case of holding devices made of sintered material, only up to 1250.degree. C.
Further, holding devices are known in which the individual device parts are connected by welding or gluing as is described, for example, in JP 08 316 158 A. However, the requirements for purity and, when gluing, also the requirements for stability in the high temperature range cannot be met due to the introduction of extraneous material. Also, known constructions in which the device parts are connected by means of joining by driving fit or press fit, as is shown in reference JP 08 102 446 A, cannot satisfy requirements for use in the high temperature range.
In order to be able to satisfy the strict requirements for purity, it is proposed in the reference JP 06 151 571 A to produce a holding device for semiconductor disks from single-crystal silicon, wherein, however, the individual device parts are connected with one another so that they can be disassembled, i.e., detached again. The required prerequisites for strength and stability in the high temperature range can also not be achieved in this case.
In view of the disadvantages of the known prior art, it is the object of the invention to provide a process of the type mentioned above and a holding device for semiconductor disks that is produced by this process, wherein the purity requirements respecting the materials used for the device parts and for the connection of parts and also the mechanical stability of the parts connection are ensured up to temperatures of 1350.degree. C.
According to the invention, in a process of the type described above, the device parts are first pre-manufactured individually from single-crystal or polycrystalline silicon and, in so doing, are provided with fitting surfaces which correspond to one another. The adjacent device parts with opposite fitting surfaces are then joined one inside or against the other and, finally, the fitting surfaces are connected with one another by diffusion welding and/or by soldering, wherein germanium powder is used as solder.
The process according to the invention makes it possible to produce a holding device for semiconductor disks which corresponds to the highest purity requirements at temperatures of up to 1350.degree. C. in the clean room, i.e., release of contaminants above the level permissible for semiconductor technology is ruled out.
The invention makes it possible, first, to produce individual device parts economically, which is advantageous particularly in the case of parts with complicated geometry. During the subsequent joining process, the device parts are permanently connected with one another by material engagement, so that a high mechanical and thermal stability of the holding device is also ensured under high-temperature conditions. This is especially important in the range of temperatures up to 1350.degree. C. and for the high weight loading due to the large number of semiconductor disks to be held.
The fitting surfaces are preferably constructed so as to be flat, but they can be curved in any desired manner; naturally, the fitting surfaces located opposite one another at different device parts must correspond or be adapted to one another with respect to curvature.
In an advantageous development of the invention, diffusion welding is carried out over a period of about three to four hours while the device parts are pressed against one another at a pressure of up to 10 N/mm.sup.2 and at an ambient temperature of 1000.degree. C. to 1300.degree. C. The device parts are preferably surrounded by atmospheric air during the welding process.
For purposes of this connection, the device parts are prepared in such a way that the oppositely located fitting surfaces of two adjacent device parts contact one another, with respect to their curvature, along the entire area provided for welding. For example, the fitting surfaces can be constructed so as to be flat and parallel to one another or also with identical curvature so as to contact one another while forming a very small gap.
In a particularly preferred development of the invention, the soldering of the device parts is carried out in such a way that powdered germanium with a grain size of less than 50 .mu.m is first introduced into a gap formed between the fitting surfaces of two device parts located opposite one another and the device parts are then connected with one another at a pressure of less than 30 mbar and at a temperature of 900.degree. C. to 1350.degree. C. The germanium acts as solder.
Instead of using germanium powder by itself, a mixture of germanium powder and high-purity water or a mixture of germanium powder and an organic oil, for example, pine oil, can be used. The addition of water or oil serves to fix the solder and improve its flowability. The additions disappear from the joint without residue during heating and have no negative influence on its strength.
Two device parts to be connected are placed opposite one another in preparation for soldering in such a way that a gap width in the range of magnitude of 0.1 mm to 0.5 mm remains between the fitting surfaces. The fitting surfaces can also be constructed so as to be flat and placed opposite one another at a distance within the above-mentioned range of 01 mm to 0.5 mm or the fitting surfaces can also be curved, wherein the curvatures at the two fitting surfaces are provided in such a way that the distance between the fitting surfaces of two device parts is not less than 0.1 mm and not greater than 0.5 mm.
The gap width can be constant along this entire surface region or a gap width can be provided between the two opposite fitting surfaces which varies within this range of 0.1 mm to 0.5 mm, which can apply to both flat and curved surfaces located opposite one another.
In a further development of the invention, flushing with argon is carried out during the soldering process. In this way, oxygen is kept away and germanium oxide formation which would negatively affect the strength of the joint is prevented.
In the development according to subclaim 2, the joint is formed of material of identical kinds or in the case of the arrangement from subclaim 3, material of different kinds, which is chemically similar to silicon and which does not permit any contamination and also satisfies requirements for stability. The proposed connection technique includes the possibility of repairing holding devices while retaining a high level of purity, which could not previously be realized with respect to defective parts. Formerly, remanufacture of at least individual parts was always necessary in the event of material breakage in devices of this type.
The invention is further directed to a holding device for semiconductor disks which is produced by the process steps mentioned above. The holding device is formed of individual device parts which are connected with one another by the process described above and between which the semiconductor disks are received so as to be oriented essentially with surfaces parallel with one another.
In particularly advantageous developments of the holding device, the device parts are outfitted with fitting surfaces which are located opposite one another from one device part to the next and which are fused together by diffusion welding according to the invention or between which germanium is introduced as solder, wherein two device parts are soldered together at their fitting surfaces.
In the following, the invention will be explained more fully with reference to embodiment examples.